1. Field of the Invention
The present invention relates to a surface acoustic wave device that includes a surface acoustic wave element such as a SAW (Surface Acoustic Wave) filter or a SAW resonator, and that has a package structure surrounding the surface acoustic wave element.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 11-88101 describes a surface acoustic wave element 102 illustrated in FIG. 8. The surface acoustic wave element 102 includes a piezoelectric substrate 111, a dielectric film 112 disposed on the piezoelectric substrate 111, and IDT (Inter Digital Transducer) electrodes 113 and 114 both disposed on the dielectric film 112.
The dielectric film 112 is a film for adjusting a fractional bandwidth of the surface acoustic wave element 102. A material of the dielectric film 112 is a metal oxide. For example, SiO2 is used as the material. From the viewpoint of ease of manufacturing, as illustrated in FIG. 8, the dielectric film 112 is formed over the entirety of a principal surface 111M of the piezoelectric substrate 111 by, e.g., sputtering. The surface acoustic wave element 102 further includes a characteristic compensating dielectric portion 121 that covers the IDT electrodes 113 and 114. The characteristic compensating dielectric portion 121 is to compensate for variations in frequency characteristics such that the frequency characteristics are not significantly changed even when the temperature of the surface acoustic wave element 102 is changed.
FIG. 9 illustrates a surface acoustic wave device 101 including the surface acoustic wave element 102 and having a package structure. The surface acoustic wave device 101 includes, in addition to the surface acoustic wave element 102, a resin member 115 disposed on the piezoelectric substrate 111. The resin member 115 is constituted by a resin support portion 115A in the shape of a frame, and a resin cover portion 115B that is disposed in a state covering the IDT electrodes 113 and 114 in cooperation with the resin support portion 115A. The piezoelectric substrate 111 and the resin member 115 define a space S that is maintained in an airtight condition to allow excitation of a surface acoustic wave in the space. The surface acoustic wave device 101 further includes via conductors 118 through which signals are supplied to the surface acoustic wave element 102.
The piezoelectric substrate 111 is made of an inorganic piezoelectric material such as lithium niobate, and the resin member 115 is made of resin. Because the piezoelectric substrate 111 and the resin member 115 are made of different materials, their linear expansion coefficients are different from each. Therefore, when the surface acoustic wave device 101 is placed in an environment subjected to temperature variations, thermal stress is generated with expansion or contraction of the piezoelectric substrate 111 and the resin member 115.
Because the resin support portion 115A and the resin cover portion 115B of the resin member 115 are both made of resin materials and respective values of the linear expansion coefficients of the resin materials are close to each other, influences of the thermal stress on both the portions are small. Accordingly, peeling-off between the resin support portion 115A and the resin cover portion 115B is unlikely to occur. On the other hand, because the linear expansion coefficients of the piezoelectric substrate 111 and the resin support portion 115A are different from each other, a shearing force is generated in a direction parallel to the interface between the piezoelectric substrate 111 and the resin support portion 115A upon the generation of the thermal stress.
In the surface acoustic wave device 101 illustrated in FIG. 9, the dielectric film 112 is disposed between the principal surface 111M of the piezoelectric substrate 111 and the resin support portion 115A. An adhesion force of the dielectric film 112 with respect to the piezoelectric substrate 111 is weak. Accordingly, if the above-mentioned shearing force is generated, peeling-off is likely to occur between the piezoelectric substrate 111 and the dielectric film 112. This results in a problem that, in the package structure of the surface acoustic wave device 101, it is difficult to maintain the interior in the airtight condition.